1. Field of the Invention
The present invention relates to a wirecut electrical discharge machine and method therefor which are operative to relatively move a wire electrode and a workpiece opposed to each other with the application of a voltage between the wire electrode and the workpiece to machine the workpiece. More particularly the invention concerns a wirecut electrical discharge machine and method therefor which are effective to improve the accuracy of inside corner machining operations during wirecut electrical discharge machining.
2. Description of the Background Art
FIG. 6a shows a conventional electrical discharge machine. In this drawing, a wire electrode 1 is connected to a machining power supply 3 and a workpiece 2 is mounted for movement by an X-table 4, which moves the workpiece 2 in an X direction, and a Y-table 5, which moves the workpiece 2 in a Y direction. Motors 6a and 6b are operative to move the X and Y tables respectively, while servo amplifiers 7a and 7b drive the motors 6a, 6b respectively. In operation, an NC program 8 and a storage device 9 which stores offset data are input to an NC unit 10, which has an arithmetic unit (not shown) to calculate machining tracks on the basis of the NC program and the offset data. A control apparatus 11 controls actual axis movements according to the machining tracks calculated by the NC unit 10.
In a conventional example, the wire electrode 1 is run by a wire electrode running device (not shown) and a pulse current is supplied between the wire electrode 1 and the workpiece 2 by the machining power supply 3 to machine the workpiece 2. The NC unit 10 calculates the machining tracks on the basis of the NC program and the offset data provided beforehand from memory or an NC tape. According to the results of the calculation of the machining tracks, the control apparatus 11 outputs move commands to the X and Y moving servo amplifiers 7a, 7b to drive the motors 6a, 6b, whereby the wire electrode 1 and the workpiece 2 are moved relatively on a two-dimensional basis to machine the workpiece 2.
The proper selection of operating voltage levels is a well known requirement for achieving high quality machining. FIG. 6b shows further details of the conventional wire electrical discharge machine. In this drawing, the components seen in FIG. 6a have the same reference number. A machining gap detection circuit 12 is used to detect an average machining gap voltage during machining. The wire 1 is supplied from a wire bobbin 10 past a tension application mechanism 17 and into the gap formed in workpiece 2. The wire then is passed through a wire runner 11 and is collected in a wire collection vessel 16. At the gap, the wire 1 is passed through dielectric nozzles 13a and 13b.
FIG. 6c shows velocity control program that is provided to the NC apparatus 10. Initially, the average voltage is detected at step 20 and then the arithmetic unit in the NC unit 10 will calculate an error between a set voltage and an average machining gap voltage per predetermined sampling time at step 21. At processing step 22, the arithmetic unit in NC unit 10 will calculate a command velocity per predetermined sampling from the error voltage found in step 21.
In operation, a velocity signal from the NC apparatus 11 causes the servo amplifier 7 to drive the servo motor 6, whereby the tables 4, 5 are moved in order to move the workpiece 2 in accordance with a desired sequence. The moving velocity during machining is changed according to the state of the machining gap. That is, the moving velocity is increased when the machining gap is wide and is decreased when it is narrow. Using this technique, the wire electrode 1 is prevented from making contact with the workpiece 2 and optimum machining is achieved. Since the gap distance during machining can be judged according to the average voltage during machining, the moving velocity is generally controlled so that the average machining gap voltage matches a predetermined set value.
Namely, as shown in FIG. 6c, the average voltage is detected at step S20. Then, a difference between a set voltage Vs that has been predetermined by the arithmetic unit and the average machining gap voltage V detected by the machining gap detection circuit 5 (hereinafter referred to as error voltage Ve) is first calculated at step S21. The arithmetic unit in the NC unit 10 then calculates a velocity component change value DF(n) defined as the function of the error voltage Ve. DF(n) is found from the product of conversion parameter K, which converts a voltage value into a velocity value, and error voltage Ve(n). Then, updated command velocity F(n) obtained as the result of adding the velocity component change value DF(n) to a previously calculated value F(n-1) is calculated. This command velocity signal is transmitted to the servo amplifier 7 to drive the servo motor 6, whereby the table 4 and the workpiece 2 are moved at a desired command velocity. Such arithmetic operations are repeated per predetermined sampling time. Accordingly, control is carried out so that the moving velocity of the workpiece may change according to the machining gap state, and machining progresses.
This type of velocity control is specific to wire electrical discharge machining wherein the control system is designed to avoid abrupt changes in velocity. Since the wire electrode is not rigid, a sharp change in velocity will cause oscillation which repeats the closure and opening of the machining gap, resulting in an inability to properly machine the workpiece.
Finishing at a corner in view of these conditions will now be considered. FIG. 7a shows changes in removal in the finishing of an inside corner, wherein A indicates a straight movement interval, B denotes a removal increase interval in which the removal increases before the corner, C represents an arc movement interval, D designates a removal decrease interval in which removal decreases before the end of the arc movement, and E indicates a straight movement interval after the corner. O.sub.1 to O.sub.4 indicate wire center positions in a corner finishing process, with 0.sub.1 representing the wire center position at the starting point of the removal increase interval, O.sub.B denoting the wire center position at any point in the removal increase interval B, O.sub.2 designating the wire center position at the starting point of arc movement C, O.sub.3 indicating the wire center position at the starting point of the removal decrease interval D, O.sub.D denoting the wire center position at any point within the removal decrease interval D, and 0.sub.4 indicating the wire center position at the end point of arc movement. L.sub.l to L.sub.4 represent removal at corresponding wire center positions O.sub.1 to O.sub.4, with L.sub.1 designating the removal when the wire center is at the position of 0.sub.1, L.sub.B designating the removal when the wire center is at the position of O.sub.B, L.sub.2 designating the removal when the wire center is at the position of O.sub.2, L.sub.3 designating the removal when the wire center is at the position of O.sub.3, L.sub.D designating the removal when the wire center is at the position of O.sub.D, and L.sub.4 designating the removal when the wire center is at the position of O.sub.4. The parameter r indicates a distance from the center point of an arc locus to a machined surface (indicated by a dotted line in the drawing) at the corner, and r' denotes the arc radius of a wire center locus at the corner.
In the drawing, while the removal in straight cutting up to the wire electrode center position of O.sub.1 (interval A) is L.sub.1, the removal at the corner inlet (interval B) increases abruptly from L.sub.1 to L.sub.2, and the corner is cut with the removal remaining increased (interval C). The removal at the corner outlet (interval D) reduces abruptly from L.sub.3 to L.sub.4, returning to the removal at the straight cutting (interval E). FIG. 7b shows an example of typical changes in removal amount (microns) in the finishing of an inside corner.
Conversely, in the finishing of an outside corner, the removal amount decreases, as shown in the example of FIG. 7c.
The conventional electrical discharge machine arranged as described above was often unable to respond in velocity to abrupt changes in removal in the finishing of a corner, whereby the machining gap changed at the corner, producing shape errors.
FIG. 8 shows a further difficulty involving the machining tracks for inside corner finishing in the conventional example. In the finishing of an inside corner, the inside corner is machined by changing a corner radius in a plurality of successive machining passes (1st to 4th cuts in the drawing) so that a machined shape has a desired radius R after the final machining pass (4th cut in the drawing). Namely, the inside corner is machined on the track to cause the corner radius in each machining pass to be a value obtained by subtracting an offset value in each machining pass from a programmed radius R (final desired radius) as indicated below: EQU Rn=R-Hn
where Rn:corner track radius in the "n"th machining
R: programmed radius
Hn: offset value in the "n"th machining.
In the conventional electrical discharge machine arranged as described above, and as disclosed in Japanese Laid-Open Patent Publication No. SHO63-105837, it was necessary to increase a corner radius gradually as the machining processes changed from roughing-in to final finishing. In this manner, the desired corner radius finally was achieved. However, in practice, there was a limit of approximately 0.2 mm to the minimum radius of an inside corner which could be finished by a wire electrode of, e.g., 0.2 mm diameter.
In view of the above difficulties in machining corners with wire cut machines, the present invention is directed to several objects.
It is, accordingly, one object of the present invention to overcome the disadvantages in the conventional design by providing an electrical discharge machine which exercises non-linear velocity control at a corner to adapt to removal changes at the corner instantly, thereby improving finishing accuracy at the corner.
It is a further object of the present invention to overcome the disadvantages in the conventional art by providing a wirecut electrical discharge machining method and a machine therefor which reduces a machinable inside corner radius sharply and can improve machining accuracy in inside corner finishing.